1. Technical Field
Apparatuses and methods consistent with exemplary embodiments relate to the use of variable impedance devices, for example Fault Current Limiter (FCL) devices, and variable impedance networks in wind turbine systems, to mitigate the effects caused by power grid disturbances such as voltage dips. Certain embodiments are configured to facilitate compliance with grid codes that are imposed upon such systems.
2. Background Art
Variable impedance devices, such as FCL devices, have recently been developed that can mitigate the increasing fault current levels during transmission and distribution in electrical power networks. Some of these developments have been accompanied by recent advances in superconductor technologies, which have resulted in, for example, super conductor fault limiter devices (SFCL) being developed. FCL devices may be divided into two main types: “permanent impedance increase” type and “condition-based impedance increase” type, which may also be referred to as “permanent impedance change” and “condition-based impedance change,” respectively. The former presents the same mode of operation in both normal and fault conditions, whereas the latter experiences a fast change of impedance during a fault.
Some examples of “permanent impedance change” FCL devices include current limiting reactors, typically air cored, high impedance transformers, which may increase the voltage level, and any other topological measures that may lower the stiffness of the system by reducing the degree of meshing, such as splitting the system into sub-grids or any kind of bus-bar splitting. The stiffness of the system is high or stiff when the source impedance is low and the available fault current is high. Thus, any topological measure which increases impedance and lowers a fault current may lower the system stiffness. According to another understanding, in a ‘stiff’ system the reduction of source terminal voltage is much less than it would be on a ‘soft’ system during a short circuit condition.
“Condition-based impedance change” FCL devices may be either passive or active, and may be connected in series or in a shunt arrangement. An example of a passive type, condition-based impedance change device is a current limiting fuse. These develop an arcing voltage high enough to effectively limit the current. Two types of current limiting fuses that may be used include stand-alone high voltage (HV) fuses and commutation fuse-based limiters.
Both “permanent impedance change” FCL and “condition-based impedance change” FCL devices could be of the “passive” or “active” type. In the case of “passive” FCL devices, the impedance change can happen naturally, as the device is stressed by the presence of fault currents, without being controlled by another device or circuit, and may therefore be called an uncontrolled, non-controlled, or passive device. In the case of “active” FCL devices the impedance change can be triggered by control circuitry, and may be called a controlled or active device. FCL devices may be connected in series with the power flow, although they are sometimes connected in a shunt fashion, diverting some of the fault currents to, for instance, the system ground. When connected in series, they may exhibit low impedance that increases rapidly during a fault event. Conversely, when connected in a shunt fashion they experience high impedance that reduces rapidly during a fault condition.
Advances in superconducting material technology have contributed to the development of FCL devices, particularly those of a “condition-based impedance change” passive type. For example, superconducting materials show a very low electric resistance, thereby simultaneously keeping the current density, temperature, and magnetic field below certain threshold values. If any of those values, typically the current density or the magnetic field, rises above the threshold in the presence of a fault current level, the material experiences a substantial impedance change that contributes to mitigation of the fault current level. This impedance increase causes additional heat dissipation, causing a subsequent temperature rise that forces the conductor further out of the superconducting region. The temperature threshold for superconducting operation is very low, in the vicinity of 4° K., or in the region of 70° K., in the case of High Temperature Superconductivity (HTS). For that reason, superconducting materials are stored in cryogenic chambers. Not all passive FCL technologies require the use of superconducting technologies.
Wind turbines based on the doubly fed induction generator (DFIG), such as doubly fed machine (DFM) shown in FIG. 1A or exciter based doubly fed machine (xDFM) shown in FIG. 1B, may be very sensitive to grid disturbances and especially sensitive to voltage dips. Overvoltages and overcurrents may occur in the rotor windings in response to abrupt drops in grid voltage that can damage the power converter if no protection is provided. A way to mitigate this transient while protecting the converter includes connecting a so-called crowbar circuit in a shunt fashion, also known as a shunt arrangement, between the rotor terminals and the converter. FIG. 1A shows a wind turbine generator 10 with a conventional active crowbar circuit 20 coupled to a back-to-back type converter 30 and a DFIG 40. The back-to-back (B2B) converter 30 includes a rotor-side or machine-side converter (MSC) 31 and a grid-side converter (GSC) 32, also known as a front-end converter or line-side converter, that are linked through a DC link or DC bus. Three phases of the rotor of the DFIG can be connected to the crowbar circuit 20 and the MSC 31, as shown in FIG. 1A. Three phases of the stator of the DFIG 40 are connected to the power grid through a transformer and three phases of the transformer are connected to the GSC 32. Upon detection of a voltage fault, the active crowbar circuit 20 short circuits the rotor of the DFIG 40 by means of a resistance element, simultaneously deactivating the MSC 31. The rotor current then flows through the crowbar 20, diverting it from the MSC 31, or according to another example directing it from the GSC 30, thereby protecting the back-to-back converter 30. The resistance imposed by the crowbar 20 helps by damping the change of flux transient, reducing the duration and magnitude of the overvoltages and overcurrents. However, the crowbar circuit 20 is costly, and bulky. For example, the crowbar may include passive and active devices, such as diodes and insulated-gate bipolar transistors (IGBTs), and a set of high-power resistors. Furthermore, the current loop established between the crowbar circuit and the converter has a high inductance and is bound to produce a significant overvoltage on the crowbar IGBT terminals at turn off. For that reason, the crowbar may incorporate resistance/capacitance (RC) damping networks and/or varistor devices to mitigate that overvoltage. The crowbar circuit may have several IGBT and resistor branches that are activated gradually as the transient evolves. Furthermore, when the crowbar turns-off, the MSC 31 may generate a voltage that contributes to mitigating the stator-flux transient created by the voltage dip.
Another configuration of a wind turbine is shown in FIG. 1B, which employs an exciter based doubly fed machine (xDFM). In FIG. 1B an exciter machine 50 is connected between the DFIG 40 and the front-end converter side of the converter 30. A conventional brake chopper 20 is connected in the DC link of the converter 30, between the machine-side converter (MSC) 31 and a grid-side converter (GSC) 32.
There are several alternatives to the shunt crowbar circuit or the brake chopper shown in FIGS. 1A and 1B, whereby the damping resistors are connected in series with the power flow. For example, FIG. 2 shows a wind turbine generator 10 equipped with a series active crowbar 21 connected between the stator of the DFIG 40 and the grid that is configured to protect the DFIG 40, the MSC 31, and the GSC 32. These options can be classified as a type of series connected, active, condition-based impedance change, FCL device. For example, a resistive FCL network 21 is connected in series with the stator windings of the DFIG 40, as seen in FIG. 2. The resistive FCL network 21 may include a resistance element Rcrow 22, which could be a set of three resistors each in parallel with bidirectional static switches 23a and 23b. These switches may be, for instance, composed of two gate turn-off thyristors (GTO) or integrated gate commutated thyristors (IGCT), connected in a back-to-back manner. A combination of a series and shunt active FCL networks may also be employed. An alternative to this is to connect the FCL device to a set of open-end terminals on the stator. These circuits may be complicated by the presence of the static switches, which, not only require the use of control and gate drive circuitry, but also need overvoltage protection during turn on.
A family of series connected, active, and/or condition-based impedance change FCL devices that are connected outside the wind turbine may exist. These devices serve multiple wind turbine generators such as wind turbine generators in a wind farm. These devices have an inverter connected in series with the main power flow, so that a voltage can be injected to mitigate the transient caused by the voltage dip. However, such systems add a high degree of complexity which affects the cost and reliability of the whole solution.
For example, the use of fault current limiter FCL of the passive, condition-based impedance change type may be used at the wind farm level. FIG. 3 shows a single-wire diagram of a super conductor FCL (SFCL) connected before the wind farm's interconnection point at the wind farm's Point of Common Coupling (PCC). The FCL employed at the wind farm level may be an SFCL 50. Specifically, the SFCL 50 is placed outside of all the wind farm's wind turbines 10 between the PCC 51 and power grid network 52. Not only does the SFCL 50 placed between the PCC 51 and power grid network 52 serve multiple wind turbines to control fault currents but it may also suppress inrush currents, when a wind farm has adopted an SFCL at the system interconnection point.
Another type of active condition-based FCL, also connected between the PCC and the power grid network, is based on a variable impedance network connected in a shunt manner to ground. Such a variable impedance network may be implemented using a variable inductor, which has a main winding for conducting alternating current and a DC control winding for conducting direct current. This type of inductor usually has the control winding wound in an orthogonal manner to the main flux in the core, so that the DC winding does not see any of the AC current, facilitating the control power supply. These two conventional approaches do not connect the FCL at the individual wind-turbine level, but rather connect the FCL at the wind farm system level.
Although these wind farm level FCLs can help mitigate adverse effects on the wind turbine supply voltage, they do not ensure the protection of the wind turbine converter equipment under grid disturbances. Also, they do not help satisfy the compliance of the wind turbine with grid codes and customer requirements, since wind turbine validation usually is a type of test performed on an individual wind turbine, at its power input terminals. Accordingly, the conventional use of FCLs at the wind farm level does not help individual wind turbines comply with such grid codes and customer requirements. As such, there is a long-felt but unmet need to satisfy these grid code and customer compliance requirements at the individual wind turbine's interface with a network or grid, as well as to protect the wind turbine's converter, avoiding the need of expensive solutions at the wind farm level.